Operation of a conventional sewing machine may be described as follows. Referring to FIG. 1, when it is intended to sew a stitch on a fabric 9, a sewing needle (not depicted in the drawings) that carries an upper thread 91 is driven to move downwardly through the fabric 9. Correspondingly, a shuttle (not depicted in the drawings) that carries an under thread 92 is driven to allow the under thread 92 to be connected to the upper thread 91. Afterward, the sewing needle is pulled upward, tightening the under thread 92 and thereby completing a stitch.
The sewing needle is then driven to move horizontally to another position. Cooperating with the movement of a fabric-feeding mechanism, the sewing needle is moved with respect to the fabric 9 in order to sew another stitch at a different location on the fabric 9.
It is noted that however, in sewing some particular patterns, horizontal displacements of the sewing needle between stitches may vary. Consequently, applying an identical tension onto the upper thread 91 in each of the stitches may result in differences in the length of the upper thread 91 supplied between the stitches. For example, when sewing a zigzag pattern as illustrated in FIG. 1, during movement of the sewing needle away from a first stitch (i.e., to a left side), an insufficient tension may lead to supply of a longer length of the upper thread 91, and consequently, excessive amount of the upper thread 91 will be left on a back side the fabric 9.